The present invention relates generally to electronic device packaging. More particularly, this invention includes a process and apparatus for securing a temporary lid to a chip substrate to aid in movement of the chip substrate during automated card assembly operations.
The increasing use of ceramic, multiple-chip modules incorporating flip-chip devices has caused correspondingly larger demands on card assembly machines which perform high-speed xe2x80x9cpick-and-placexe2x80x9d operations. The ever-increasing range of possible chip layouts and encapsulation schemes exacerbates those demands. Therefore, the card assembly machines have had to become, and continue to become, better suited to perform pick-and-place operations on chip modules during card assembly.
Typically, in the case of common, plastic, overmolded, dual-in-line packages (DIPs), automated picking of the components is performed by a vacuum probe. The vacuum probe attaches to the card package by contacting the flat, plastic, outer surface of the chip. The difference in pressure between the ambient atmosphere and the inside of the vacuum probe (with the chip surface sealing the probe opening) keeps the package attached to the probe until the vacuum is released, which releases the package from the tip of the vacuum probe. FIG. 1 illustrates this process according to the prior art, where vacuum probe 110 is shown attached to the module 100 through contact with chip 120. The module 100 is typically referred to as a capless chip module. The module 100 of FIG. 1 is shown as already placed and seated on a printed circuit board 130.
FIG. 1 also shows additional components of a typical capless chip module 100 seated on top of a printed circuit board 130, according to the prior art. Printed circuit board 130 supports a plurality of solder balls 140 which in turn support a substrate 150 (often, but not necessarily, ceramic). If the substrate is ceramic, the substrate 150 and its solder balls form what is known as a ceramic ball grid array (CBGA). Alternatively, a ceramic column grid array (CCGA) (not shown) technology could be used. Typically, the chip 120 is secured to the substrate 150 via a plurality of controlled collapse chip connection (C4) balls 145.
The substrate 150 could also have one or more electronic devices 160 attached to it via the C4 balls 145. Examples of such electronic devices include: decoupling capacitors, resistors, capacitors, and inverters. In addition, these devices could be attached to the substrate not by C4 balls 145, but rather by surface-mountable solder (not shown). For reliability reasons, the C4 balls are encapsulated with a polymeric underfill material 170. Where multiple chips and electronic devices are combined oh one substrate, as in FIG. 2, the package is typically referred to as a multiple chip module (MCM).
In a typical card assembly manufacturing process, a pick-and-place tool picks up each module 100 that is to be joined to the printed circuit board and places it in the proper location. The board 130 and these placed modules 100 are then heated in a card assembly heating apparatus and the solder balls 140 are xe2x80x9creflowedxe2x80x9d causing the connection of module 100 and printed circuit board 130 to occur.
As MCMs become larger and more specialized, automated pick-up and placement of cap-less modules become more difficult because the chip center lines may be located on an asymmetric grid with respect to the center line of the module. In other words, with a multitude of chips and components on the substrates (as is the trend), there may not be a clean, flat, and smooth surface available in the middle of the module to which the vacuum probe can attach. Non-flat surfaces having encapsulants, glob top, or other polymeric materials also create problems for pick-up tools. Unless the card assembly picking tooling is automated and flexible enough to locate a chip surface off the packaging center (even in a high-speed mode), tool efficiency will suffer greatly, because many chips will simply not be picked up without changes being made in the tooling each time a module type is run. Moreover, many automated pick-and-place tools cannot move off module centers and such tools cannot practically be modified (due, in part, to cost constraints). The vacuum probe method is most economical and efficient for packages having a relatively flat top surface. It is often not well suited, however, to making dynamic, offset motions needed to accommodate off-center device locations, nor is it effective in spanning multiple chips, unless a custom pickup probe is fitted to each package type.
FIG. 3 illustrates another process for moving chip modules 100 during production according to the prior art, where the center of the lid is used for pick and place operations. The module shown in FIG. 3 is typically referred to as a capped module, because a module cap 210 is used. Here, vacuum probe 110 is attached not to a chip, but rather to module cap 210. Module cap 210 is attached to substrate 150 of module 100 via a cap seal 220 (adhesive for non-hermetic modules and solder or glass for hermetic modules). Module cap 210 is attached to chips 120 via a semi-liquid or paste-type, thermally conductive material 230. It is important that cap 210 be attached to each chip 120 through a thermally conductive material because the chips would otherwise overheat during operation. Module cap 210 is typically metal and presents a clean, smooth, flat surface to vacuum probe 110.
In the package assembly shown in FIG. 3, because the cap 210 is attached to the substrate 150 via a cap seal 220,.the attachment is substantially permanent. If the cap 210 is removed from the substrate 150 at some later step for rework in the manufacturing process, the substrate may be damaged, or seal material may be left behind. Such a condition increases the risk of damage when additional components are subsequently reworked to the substrate 150. The trend is away from module caps 210 and toward non-hermetic packaging methods.
In the device as illustrated in FIG. 3, the chips 120 are attached to the module cap 210 via thermally conductive material 230 which transfers heat from the chips 120 to the module cap 210 where it can be further dissipated. The necessity of a thermally conductive material 230 associated with capped modules adds to the cost of reworking and adds a higher rate of unreliability to the package.
The deficiencies of the conventional manufacturing techniques show that a need still exists for a process and apparatus which will accurately and reliably attach a temporary, removable lid to a chip carrier to allow vacuum pick-up by high-speed, automated assembly tools. Therefore, one object of the present invention is to provide an apparatus and process for attaching a temporary, removable lid to a chip carrier containing one or more microelectronic devices using a mechanical clipping lid which attaches to the substrate via friction that allows efficient vacuum pick-up by high-speed, automated assembly tools.
Another object of the present invention is to provide an easily removable lid attachment which may be removed after the need for vacuum probe attachment is no longer necessary in the manufacturing process. This object would include a removable lid which does not, upon removal, result in mechanical or structural damage to the devices that would impede subsequent heatsink attachment schemes which may involve the use of adhesive compounds.
Still another object of the present invention is to provide a process and apparatus that will absorb thermally induced strain without damage to the chip carrier or associated devices. Still yet another object of the present invention is to provide an apparatus that will allow pressure to be applied to the module during testing and burn-in without damage to the module.
Yet another object of the present invention is to provide a process and apparatus for ensuring mechanical and operational integrity of the bond between devices on the substrate and a heatslug, especially under typical shipping and use conditions such as gravity, mechanical shock, vibration, high temperature, humidity, and repeated thermal expansion and contraction cycles due to temperature cycling during operation.
To achieve these and other objects, and in view of its purposes, the present invention provides a process and apparatus for high speed pick-up and placement of a chip module during card assembly. A mechanical clip is temporarily attached to the chip substrate and covers the components attached to the substrate. The top surface of the mechanical clip is a flat, clean, and smooth metal surface, providing a smooth and clean contact area for a vacuum probe to pick up the module. During manufacturing steps which require the movement of the module, the clip is maintained in place. After reflow is complete and the module is attached to a printed circuit board, the clip may be removed, either to be discarded or reused.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.